I still have no idea what possessed Olympus to make such a slow zoom. Sharp as it may be (it better be. The spec is so low that if it cannot make it sharp wide open something is very wrong), the usefulness is another matter. It is really a daylight lens and utterly useless at night without a tripod.
It's all about size.
When you're selling people on a small system, you don't show them huge equipment. Four-Thirds was supposed to be a small system and yet, the good glass is huge. Sure, it's smaller at the 135 format-equivalent focal length, but it's still huge and heavy. People even complain about the 50-200mm being heavy, though it's quite light and small.
Panasonic already commented about why their new 12-35mm and 35-100mm lenses will be f/2.8 and not faster--size.
However, if Olympus (or Panasonic) are courting professional users, they'll have to show bigger, faster lenses.